The present invention relates to a combination of a fluidized bed and a cyclone for separating out particulate material and returning it to the fluidized bed.
In a fluidized bed, some type of particulate material is fluidized by passing gas through it. In general, the particulate material may be a catalyst, a fuel, or a reactant. When the gas is passed through the particulate material, it raises the level of the material in the vessel from the static bed level to the fluidized bed level. The gas tends to carry some particulate matter with it, and it is desirable to separate the particulate matter out of the gas and return it to the fluidized bed before it passes out of the process, both because the particulate material may be expensive, and because it would tend to pollute the atmosphere or foul downstream equipment if it were not collected and returned to the process.
In order to separate the particulate material out of the gas, cyclones are often used. The particulate-laden gas enters the cyclone in a downwardly-rotating manner. The particulate material is thrown outwardly and downwardly, while the relatively clean gas reverses direction and exits the top of the cyclone. Most of the particulate material then passes through a leg of the cyclone and back down into the fluidized bed.
In the prior art, there is usually a flapper valve at the bottom of the leg of the cyclone to prevent the fluidized particles from travelling up from the bed through the bottom of the leg. When enough particulate material goes down the leg and rests against the flapper valve, the weight of the material causes the flapper valve to open, dumping the particulate material into the fluidized bed. There have been many problems with the flapper valves failing to function properly, resulting in particulate material plugging up the dip leg, which results in excessive loss of bed material and possible shut-down of the system.
In the prior art, there is always some sort of gas distribution system near the bottom of the fluidized bed in order to fluidize the bed. This gas distribution system may be expensive to install and maintain. The cyclone system may be mounted internally or externally to the fluid bed vessel.